I already set-up my root password in Ubuntu. But for some reasons now I need to set it back to default as when Ubuntu is freshly installed. I no more need my set root password. How can I set it back to default? I know my root password. When I supply with empty password to set it back to default it says "Cannot be empty"
To lock the root account, making it unable to login with a password, use this command:
sudo passwd -l root
It will retain whatever password you have set, but it will be unusable for all purposes.
EDIT: This will be useful for any account, besides
To unlock the password for the account you previously locked, use this:
sudo passwd -u root
BTW: as was commented, you shouldn't use the
root account, unless you really know what you're doing.